1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly to a film reading apparatus for reading through light transmission the image printed in a silver salt film.
2. Related Background Art
It has been proposed heretofore to photoelectrically read images recorded in books, magazines and so on by using a CCD image sensor or the like to thereafter transmit the images to a remote location, electrically edit them, store them in an electronic file or the like, or perform other processings.
It has also been proposed to photoelectrically read images recorded in a film such as a 35 mm film, microfilm and so on by using a CCD image sensor or the like. The present applicant has already proposed apparatus for reading images recorded in a film, for example, as in U.S. Pat. Nos. 4,674,126, 4,700,237, 4,762,985, 4,837,450, 4,825,065, 4,933,983 and U.S. application Ser. No. 419,702 (filed on Oct. 11, 1989).
An image in a silver salt film includes the information of very broad dynamic range as compared with a reflection image of an ordinary printing matter or the like. Printing matters has the image density range of about 0 to 1.5, whereas the transmission density range of a film is about 0 to 3.0 which is about two times as broad as that of printing matters.
If an image in a film has a limited density range within the film transmission density range (0 to 3.0), it is common that the image only within the limited density range is read and converted into image signals. This is particularly the case for a negative film.
Next, a method of variably changing the density range will be described with reference to FIG. 1.
In the graph shown in FIG. 1, the first quadrant illustrates the relationship between the density of a subject to be taken with a film and the density of the image printed on the film. A curve 31 represents the film characteristic which shows that the black and white are reversed.
The second quadrant illustrates the relationship between the film density and an output value obtained by reading the image and performing A/D conversion and logarithmic transformation. A straight line 32 indicates that the film density 0 to 3.0 is reversed in reading the image.
The third quadrant illustrates how the output value after logarithmic transformation is changed to a signal having only a specific density range.
The fourth quadrant illustrates the relationship between the final image signal and the original camera subject density.
If a subject has a low contrast and flat image, the subject density range is restricted to .DELTA.D.sub.0, whereas if a subject has a high contrast and large luminance difference, the subject density range is restricted to .DELTA.D.sub.0 ".
The subject density ranges .DELTA.D.sub.0, .DELTA.D.sub.0 ', and .DELTA.D.sub.0 " correspond to the film density ranges .DELTA.D.sub.f, .DELTA.D.sub.f ' and .DELTA.D.sub.f ". The image of a subject is read by restricting the film density range to one of three ranges .DELTA.D.sub.f. .DELTA.D.sub.f ' and .DELTA.D.sub.f ". Such change is carried out by using one of the straight lines 33, 34 and 35 shown in the third quadrant in FIG. 1.
The changed output signal is represented by one of the straight lines 36, 37 and 38 shown in the fourth quadrant.
The output after A/D conversion does not correspond to the film density D, but to the film transmittance T. The relationship between the film density D and the film transmittance T is given by the following equation (1): EQU D=-log T (1)
The change in image signal by using the straight line 32 in the second quadrant shown in FIG. 1 and one of the straight lines 33, 34 and 35 can be efficiently performed at the same time by using a logarithmic transformer 109 shown in FIG. 2.
FIG. 2 shows a film reading apparatus employing such efficient processing.
In FIG. 2, reference numeral 103 represents a film to be read, 101 a lamp, and 102 a condenser lens, these elements constituting transmission type illumination means which transmission-illuminates the film 103 through the Kohler's illumination. Reference numeral 104 represents a focusing lens for focusing an image transmitted from the silver salt film 103 onto the surface of photoelectric conversion means in the form of a photoelectric conversion element 105 such as a charge storage type CCD line sensor, area sensor or the like. If a CCD line sensor is used as the photoelectric conversion element 105, the silver salt film 103 or the CCD line sensor is moved by a driver mechanism (not shown) to read the entire area of an image.
Reference numeral 106 represents an amplifier which amplifies an output from the photoelectric conversion element 105 to a predetermined output level. Reference numeral 107 represents a sample/hold circuit which samples and holds the an output signal from the amplifier 106 on the pixel unit basis. Reference numeral 108 represents an A/D converter which converts an output signal from the sample/hold circuit 107 into a digital signal. Reference numeral 109 represents a logarithmic transformer serving as logarithmic conversion means which stores a plurality of logarithmic conversion tables for different reading density ranges and operates to select a conversion table corresponding to one of the curves 41, 42 and 43 shown in FIG. 3 in accordance with a density range select signal, i.e., address signal (digital value) to thereby logarithmically transform the digital signal corresponding to the transmittance T from the A/D converter 108. The logarithmic transformer 109 outputs a final image signal.
The curves 41, 42 and 43 shown in FIG. 3 correspond to the lines 33, 34 and 35 shown in FIG. 1.
The graph shown in FIG. 3 is used for transformation between digital signals. As seen from FIG. 3, the input dynamic range of the curve 41 is narrower than that of the curve 43 so that bits are likely to be missed during signal transformation, resulting in a problem that the quality of a final image may be lowered.
As a means for solving such a problem, there is known a method wherein logarithmic transformation is carried out in an analog fashion by using a logarithmic amplifier or the like prior to the A/D conversion. Although this method solves the above problem of lowering the final image quality, there arises a new problem that the apparatus becomes bulky.